The invention relates to an amplifier circuit for delivering a signal which varies in amplitude as well as in phase or frequency, preferably a radio frequency amplifier circuit.
The invention relates as well to a radio transmitter or a cellular telephone comprising an amplifier circuit in their aerial output stage.
Further, the invention relates to a method for controlling a radio frequency output amplifier circuit in a radio transmitter, and to the use of said method in a mobile radio transmitter.
From the article: Leonard R. Kahn: xe2x80x9cSingle-Sideband Transmission by Envelope Elimination and Restorationxe2x80x9d, Proceedings of the I.R.E., 1952, pp. 803-806, a Single-Sideband (SSB) Transmitter is known, wherein the phase and amplitude components of an input signal are separated and amplified independently before being mixed in a final output stage.
It is a disadvantage in this transmitter that modulating the amplitude at a high power level tends to generate considerable amounts of spurious signals.
From the article: V. Petrovic and W. Gosling: xe2x80x9cPolar-Loop Transmitterxe2x80x9d, Electronics Letters, 10th May 1979 Vol. 15 No. 10, pp. 286-288, a Polar Modulation Feedback Linearisation System for an SSB Transmitter is known, wherein the modulated output from a Radio Frequency Power Amplifier (RF PA) is being controlled by means of two closed loops, each having the same modulated Intermediate Frequency (IF) Signal as the controlling variable. A first loop controls the Amplitude Modulation (AM) or the xe2x80x9cenvelopexe2x80x9d of the RF output signal relative to the IF signal, and a second loop controls the Phase Modulation (PM) of the RF output signal relative to the IF signal. Polar representation of the RF signal vector is used in the article, the vector angle representing the signal phase and the vector magnitude representing the signal amplitude.
It is a disadvantage in systems of the two types mentioned, that the gain of known RF power amplifiers is varying with the power level. This entails poor linearity at high power levels and in the latter system a poor stability of the closed loop. These deficiencies often cause spurious signals or sidebands to be emitted.
The object of the invention is to provide an amplifier circuit for amplifying and delivering signals which are amplitude modulated as well as phase or frequency modulated, the amplifier having improved stability and linearity throughout the entire power level range.
It is a further object of the invention to provide an amplifier circuit of said type with an improved power efficiency.
In particular for transmitter power amplifier circuits used in cellular telephones (mobile phones), stability and linearity are of prime importance in order to utilise efficiently the very limited frequency bands available for this service. Also, a high power efficiency is very important in mobile equipment, in order to obtain a long operating time from a given battery capacity.
In a first aspect, the invention provides an amplifier circuit for receiving one or more input signals and delivering an output signal which varies in amplitude and in phase and/or frequency, the circuit comprising a power amplifier producing said output signal, a first feedback circuit providing feedback of the amplitude value of said output signal, and wherein said feedback circuit has its gain controllable in dependency of one or more properties of the input signals, the output signal or both, characterised in that the circuit is arranged to differentiate between output power levels to achieve constant gain in the amplitude loop.
Preferably, the circuit is arranged to control the amount of signal feedback to the input of the amplifier according to the output power level.
Preferably, the circuit is arranged to increase the amount of signal feedback to the input of the amplifier at high output power levels, and decrease the amount of signal feedback at low output power levels.
Preferably, the circuit comprises a second feedback circuit providing feedback of the phase or frequency of said output signal, said feedback circuits constituting a closed feedback loop together with said power amplifier.
In one embodiment, the invention provides an amplifier circuit for receiving one or more input signals and delivering an output signal which varies in amplitude and in phase or frequency, the circuit comprising a power amplifier producing said output signal, a first feedback circuit providing feedback of the amplitude value of said output signal, and a second feedback circuit providing feedback of the phase or frequency of said output signal, said feedback circuits constituting closed feedback loops together with said power amplifier, the object mentioned is met in that at least one of said feedback circuits has its gain controllable in dependency of one or more properties of the input signals, the output signal, or both.
Hereby, the phase or frequency linearity as well as the amplitude linearity for the amplifier circuit are substantially improved. This improvement in linearity may advantageously be exploited to achieve better signal qualities such as lower levels of spurious signals, thus enabling e.g. a closer channel spacing or a higher modulation factor without inter-channel cross-talk.
In addition, said improvement may be exploited by the use of power amplifiers having higher power efficiencies, such power amplifiers most often having a marked non-linear behaviour. The use of power amplifiers with higher power efficiencies is generally favourable, but especially so in mobile equipment such as cellular telephones.
It is generally preferred that the output signal is an RF signal. Hereby, the advantages of the amplifier circuit are utilised efficiently in that the amplifier circuit enables a radio transmitter output signal of high quality to be produced, this signal meeting the demands for radio transmissions in to-day""s very crowded radio frequency spectrum.
Preferably, the gain of one or more of said controllable gain feedback circuits is controlled by a gain control signal.
Hereby, said gain may be controlled from a sub-system such as e.g. a digital signal processor.
In a preferred embodiment, the first feedback circuit has its gain controllable in dependency of the amplitude of the output signal.
This will enable stabilisation of the amplitude feedback loop gain in the case where the gain of the power amplifier is varying with the output signal amplitude. This is most often the case in power amplifiers with a high power efficiency. In this situation, a substantially constant, high loop gain will be achievable without risk of the varying gain of the power amplifier leading to instabilities in the amplitude loop.
It is preferred that the second feedback circuit constitutes a phase lock loop together with said power amplifier.
This will enable a high phase linearity of the output power amplifier, which will tend to lower any phase distortion arising from imperfections in the power amplifier.
xe2x80x9cCross-talkxe2x80x9d from the amplitude modulation to the phase or frequency modulation (or: phase or frequency distortion, respectively, generated by the amplitude modulation) in amplifiers processing compound modulated signals will always be present to some small extent, due to the fact that the shift of the amplifier gain introduced by the shift of the modulated amplitude will in itself affect the momentary phase or frequency, respectively, of the signal.
Substantially cross-talk from the amplitude modulation to the phase or frequency which will normally be encountered when using a power amplifier with a poor amplitude linearity for amplifying signals that are both amplitude and phase or frequency modulated, is however avoided in the embodiments of the invention with both an amplitude feedback loop and a phase locked loop, because the phase distortion generated from the amplitude modulated signal by the amplitude linearity defects of the power amplifier are cancelled by the phase linearity feedback of the phase locked loop.
This is so because the phase distortion is generated inside of the phase locked loop, and thus in a part of the circuit which is controlled by the phase locked loop.
It is particularly preferred that one or more of said input signals are digital signals.
This will enable the amplifier circuit of the invention to be fed with digital modulation information directly from a digital signal processing system, thus simplifying the circuitry of e.g. a radio transmitter comprising the amplifier circuit of the invention.
It is a further object of the invention to provide a radio transmitter having improved amplitude linearity as well as phase or frequency linearity in its aerial output signal.
This object is met by a radio transmitter comprising an amplifier circuit according to the invention in its aerial output stage.
By the radio transmitter comprising an amplifier circuit according to the invention in its aerial output stage, said first feedback circuit providing feedback of the amplitude value of the output signal, and said second feedback circuit providing feedback of the phase or frequency of said output signal, will provide improved amplitude linearity as well as phase or frequency linearity in the aerial output signal from the radio transmitter.
In addition, at least one of said feedback circuits having its gain controllable in dependency of one or more properties of the input signals, the output signal, or both, will enable operation of the amplifier circuit with a constant, high loop gain in the gain controllable loop or loops. This will further improve said linearities.
It is preferred that the radio transmitter comprises a digital signal processing system delivering one or more input signals to said amplifier circuit.
Processing or generating these input signals within a digital signal processing system entails important advantages in that such digital signal processing systems can be large scale integrated, thus reducing cost, weight and power consumption, and in that digital signal processing tends to be more precise and less susceptible to noise.
It is preferred that a digital signal processing system delivers a gain control signal to the amplifier circuit.
Hereby, calculation of a desired gain may readily be made in the digital signal processing system, where adequate computing power is most often already present.
In this connection, it is preferred that the gain control signal is generated by the digital signal processing system from one or more of said input signals.
As these signals are already generated within the digital signal processing system for use as input signals to the amplifier circuit of the invention, derivation of said gain control signal therefrom is advantageously done within said system.
In a particularly preferred embodiment, the digital gain control signal is generated within said digital signal processing system by means of an internal look-up table.
An internal look-up table constitutes an advantageous and efficient means for providing the gain control signal from the input signals in real time processing.
It is a still further object of the invention to provide a cellular telephone having improved amplitude linearity and phase or frequency linearity in its aerial output signal.
In a cellular telephone, this object is met in that the telephone comprises a radio transmitter according to the invention. This entail corresponding advantages as explained above with reference to the radio transmitter.
Another object of the invention is to provide a method for controlling an amplifier circuit of the types mentioned.
In a method for controlling a radio frequency output amplifier circuit in a radio transmitter, said output amplifier circuit receiving one or more input signals and delivering an output signal which varies in amplitude and in phase or frequency, the circuit comprising a power amplifier producing said output signal, the method comprising the steps of:
feeding back the amplitude of the output signal of the power amplifier via an amplitude feedback path to circuitry comparing the fed back amplitude to a value of at least one of said input signals, thereby providing an amplitude feedback loop around the power amplifier,
feeding back the phase or frequency of the output signal of the power amplifier via a phase or frequency feedback path to circuitry comparing the fed back phase or frequency to a value of at least one of said input signals, thereby providing a phase or frequency feedback loop around the power amplifier, said object is met in that the method further comprises the step of:
controlling a gain in at least one of said feedback paths in dependency of one or more properties of the input signal, the output signal, or both, thereby controlling the loop gain in the corresponding feedback loop.
By this measure, the phase or frequency linearity as well as the amplitude linearity for the output amplifier circuit are substantially improved. These improvements in linearity may advantageously be exploited to achieve better signal qualities such as lower levels of spurious signals, thus enabling e.g. a closer channel spacing or a higher modulation factor without inter-channel cross-talk.
In addition, said improvement may be exploited by the use of output power amplifiers with higher power efficiencies, such output power amplifiers most often being very non-linear. The use of power amplifiers with higher power efficiencies is generally favourable, but especially so in mobile equipment such as cellular telephones.
It is generally preferred that one or more of said input signals are digital signals.
This will enable the output amplifier circuit to be fed with digital modulation information directly from a digital signal processing system, thus utilising the inherent advantages associated with digital signal processing in terms of large scale integration of circuits and flexibility in programming such systems, and in terms of more favourable precision and noise characteristics.
In a preferred embodiment, the gain of said amplitude feedback path is controlled in dependency of the amplitude of said output signal.
In this way, an important control of the resulting loop gain is attained, rendering it possible to keep a continuously high loop gain and thereby a high amplitude linearity by counteracting variations in the amplitude gain of the output power amplifier.
In a further preferred embodiment, the gain of said amplitude feedback path is controlled as a predetermined function of the amplitude of said output signal, and said function is determined to counteract variations in the gain of said power amplifier with the amplitude of said output signal, whereby a loop gain of substantially constant value is obtained in the amplitude feedback loop despite said variations.
When variations in the amplitude gain of the output power amplifier with the amplitude of the output signal are known, the gain in the amplitude feedback may according to the invention be controlled to obtain a continuously high loop gain.
In a particularly preferred embodiment, a digital representation of said function is stored in a digital memory, preferable a look-up table in a digital signal processing system.
Hereby, an advantageous, fast and cost efficient read-out of the function is had, resulting in fast and simple control of the loop gain.
Finally, it is an object of the invention to provide a mobile radio transmitter such as a cellular telephone having improved amplitude linearity and phase or frequency linearity in its aerial output signal.
According to the invention, this object is met by the use of the method according to the invention in a mobile radio transmitter, preferably a mobile radio transmitter in a cellular telephone.
Said use entails advantages corresponding to those mentioned above with respect to the method of the invention. Said advantages are particularly relevant in the case of mobile radio transmitters as they permit reductions in size, weight, cost and power consumption of said transmitters. In the case of cellular telephones, said cost reductions are important owing to the heavy competition in the market for cellular telephones.